Drop support for semihosting

We don't use semihosting in any of our test configs at the moment so
there's a risk this code might get broken without us notifying it.
It seems better to reintroduce it if and when we actually need it.

Change-Id: Iae84e3be034cc3da0248954aa5a1029ddd50aabb
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>